The field of the invention relates to industrial borescopes and measurements taken therewith.
Borescopes are often used in industrial applications to inspect objects within machines or devices which are difficult or impractical to disassemble for inspection, such as aircraft engines or power generation turbines. U.S. Pat. No. 5,633,675 describes a borescope device having a mechanism for generating and projecting a shadow on an object of interest. The shadow has a known projection geometry relative to the field of view. The position of the shadow in captured images provides object distance and magnification information which enables the determination of the size of other visible elements on the object of interest being inspected with the borescope. Other measurement methods such as the stereoscope approach described in U.S. patent also allow such size determinations.
Referring now to FIG. 1 there is shown a borescope imager head 10 housed in a small diameter housing 14. The borescope imager head 10 has a fiber optic bundle 12 for conveying light from an illumination source outside the head, and a video image sensor 18 with associated optics 20 placed distally at least a given distance from the image sensor. The light from the tip 42 of the fiber optic bundle 12 passes through an optically transparent illumination window 32 into which is set, etched, drawn or otherwise affixed an opaque object 16 that can cast a shadow either on the target or in the plane of the target. As known in the art, the video image sensor 18 comprises a CCD imager 22, a hybrid circuit 26 connected thereto by well known means such as wires, and a cable 30 for relaying the video image detected through an optically transparent optic window 33 to a monitor or other device, typically via a video processing system. Video processing systems can include A/D converters, digital signal processing and D/A conversion to generate a signal suitable for analog monitors.
Borescope images can reveal that portions of an apparatus under inspection are damaged, such as by missing portions of the apparatus. Turbine blades, for example, do wear or can break, so that the end portions of the blades are sometimes broken off or worn from the original size and shape. It is advantageous when using a boresope to inspect an apparatus to also be able to determine the size of the missing or worn portion of the apparatus under inspection. The missing length of one or both original edges must often be known for compliance with standards for wear and damage. Current methods for determining missing dimensions do not address accurately determining the length of the missing original edges. Current methods typically assume that edges of a blade are perpendicular and in a single plane, which is not usually the case, so that measurement error is inherent when using current methods. It is also difficult to determine a missing area of a blade or other object corner, since two edges must be extrapolated and combined with the remaining missing portion edge to obtain the border of the missing area. Accurately determining the bounds of a missing portion is not readily done using current methods.
U.S. Pat. No. 6,945,931 describes several methods for calculating the missing area of a turbine blade using a borescope. The methods require determination of at least one plane relative to the blade, combined with a second plane or line, that is used to establish the boundary of the original, undamaged blade. Vectors are used with points on the blade to establish the plane perpendicular to the blade top edge. Complex calculations are required simply to establish the position of the borescope relative to the blade for taking a measurement. The '931 patent provides methods for determining area by dividing the boundaries of the missing area into smaller, geometric shape samples, for which samples area calculations are more easily made and then summed to produce a whole missing area value.
The above-referenced patent describes a complex method involving the user placement of numerous cursors to determine a missing corner area. Many maintenance specifications for turbine engines set limits on the length of material missing along the two edges that would intersect at the corner rather than the missing area. Thus, the referenced patent does not provide the information needed by inspectors, and it requires the placement of more cursors than are needed to determine the necessary missing length information thus reducing the efficiency of such measurements. Accordingly, a need exists for an improved measurement system which more quickly and efficiently provides a more accurate missing corner measurement, in particular, one which provides a measurement of the length and/or depth of the missing area bounds.